Poly-crystalline silicon is a very promising target material when deposited on a flexible substrate for fabricating solar cells and thin film transistors. Thin-film transistors have huge markets in flat panel television or liquid crystal displays that are used in a number of electronic devices. The large market for these devices makes it desirable to develop a low cost method of making these and other devices that utilize a high temperature treatment for the target material, but where the substrate upon which the target material resides does not tolerate high temperatures well.
Conventional doping of semiconductor material includes diffusion and ion implantation. In situ doping of semiconductor material during deposition is also another doping technique. Doping by diffusion is a high temperature process; hence, it is not suitable for fabricating devices of glass or plastic substrate. Ion implantation is a technique where dopant is injected directly into the material lattice. In situ doping during deposition creates a hetero-junction and can cause interface problem.
Polycrystalline silicon can be prepared by crystallization of amorphous silicon by various techniques. Polycrystalline silicon can be deposited directly onto the substrate by low pressure chemical vapor deposition (“LPCVD”). Solid phase crystallization is another technique that has been used. The main problem in these techniques is the processing temperature. The usual deposition temperature of this technique is around 600° C. The grain size of the deposited film is usually in the range of 30 to 100 nm for LPCVD and in the range of 200 nm to 400 nm for solid phase crystallization. The processing temperature eliminates the use of many plastic substrates that are otherwise ideal for these applications.
Researchers have studied laser crystallization. The major disadvantage of laser crystallization is its low throughput due to the small laser spot size, which is not suitable for large area device like solar cells. Because of the pin-point heating, the laser has to be moved many times, perhaps thousands of times for a large substrate. Movement of the laser has to be precise to cover all of the target material and obtain a treated target material with few defects. This process is very expensive and time-consuming. If the laser is not aimed accurately, there will be areas of unconverted target material and a large portion of impurities.
Other possible treating methods include hydrogen-induced crystallization and metal-induced crystallization. Hydrogen induced crystallization involves high temperature deposition and is not suitable for plastic substrates. Metal induced crystallization also requires high temperatures of more than 180° C. for fabricating good quality poly-silicon. Another major problem of this technique is metal contamination of the film. Pt/Al alloys can be achieved by high temperature annealing.
Energetic materials such as explosives have been examined by several researchers to crystallize various bulk amorphous materials. Some explosives are unsuitable as they produce sufficient energy to destroy the device to which they are attached. Others are not self-propagating so that, when used in thin layers, combustion of one section of the layer does not automatically ignite adjacent parts of the film.